The Old World monkeys or Cercopithecidae are a group of primates, falling in the superfamilyCercopithecoidea in the cladeCatarrhini. From the point of view of superficial appearance, they are unlike apes in that most have tails (the family name means "tailed ape"), and unlike the New World monkeys in that their tails are never prehensile. Technically, the distinction of catarrhines from platyrrhines depends on the structure of the nose, and the distinction of Old World monkeys from apes depends on dentition.

Several Old World monkeys have anatomical oddities. The colobus monkeys have a stub for a thumb; the Proboscis Monkey has an extraordinary nose while the snub-nosed monkeys have almost no nose at all; the penis of the male Mandrill is colored red and the scrotum has a lilac color, while the face also has bright coloration like the genitalia and this develops in only the dominant male of a multi-male group.

The Old World monkeys are native to Africa and Asia today, but are also known from Europe in the fossil record. They include many of the most familiar species of non-human primates.